Method for producing an oxide layer on metallic elements

ABSTRACT

In a known method for producing an oxide layer on metal parts, the metal parts are heat-treated in a treatment chamber during a carburization phase at temperatures below 1100° C. [2012° F.] in an atmosphere containing carbon monoxide and hydrogen and then, during an oxidation phase, they are oxidized at an oxidation temperature in the range from 750° C. to 950° C. [1382° F. to 1742° F.] in an atmosphere where a PH 2 O-to-PH 2  ratio between 0.3 and 10 has been established by feeding an oxidant into the treatment chamber, a process in which an oxide layer is formed, whereby the oxygen partial pressure is determined by means of an oxygen probe and regulated at least during the oxidation phase. In order to improve the known method in terms of the reproducibility of the formation of corrosion-resistant coatings that cover and adhere well on metal parts containing iron and so as to improve the cost effectiveness of the process, it is proposed according to the invention that the oxygen partial pressure be regulated in such a way that the oxygen probe indicates a probe voltage in the range from 890 mV to 940 mV.

The invention relates to a method for producing an oxide layer on metalparts in which the metal parts are heat-treated in a treatment chamberduring a carburization phase at temperatures below 1100° C. [2012° F.]in an atmosphere containing carbon monoxide and hydrogen and then,during an oxidation phase, they are oxidized at an oxidation temperaturein the range from 750° C. to 950° C. [1382° F. to 1742° F.] in anatmosphere where a PH₂O-to-PH₂ ratio between 0.3 and 10 has beenestablished by feeding an oxidant into the treatment chamber, a processin which an oxide layer is formed, whereby the oxygen partial pressureis determined by means of an oxygen probe and regulated at least duringthe oxidation phase.

The quenching and tempering, case hardening or carbonitriding, etc. ofserially-produced parts is carried out in furnace installations throughwhose treatment chamber inert and/or reactive gases flow. In thisprocess, the gas atmosphere in the treatment chamber of the furnace isset in such a way that a clean and bright metal surface is obtained.Oxidation is not desired.

With the use of nitrogen in combination with reactive components (forexample, methanol cracking gas, an endothermic atmosphere or the like),the parts are heated up to the appertaining austenitizing temperature,then thermochemically treated and hardened by quenching in oil, water orliquid salt.

In a subsequent second heat-treatment step, the requisite hardness isset by annealing the hardened parts. The annealing takes place attemperatures between 100° C. and 550° C. [212° F. and 1022° F.] in anair or nitrogen atmosphere.

Under these annealing conditions, the iron can oxidize to form magnetite(Fe₃O₄). The oxide layer formed does not exhibit sufficient layerthickness to provide a visually attractive and uniform decorativesurface.

Therefore, the heat-treatment procedure is followed by coating processesthat produce a visually attractive and uniform decorative layer on thesurfaces. These coatings are often applied by means of demandingwet-chemical processes (burnishing, phosphatization, etc.). Acids, lyes,highly concentrated salt solutions and washing water are employed insuch cases and they generate large quantities of hazardous waste andemissions. These coating processes entail considerable processing effortand additional costs.

As an alternative to this, in a process of the above-mentioned typeaccording to German patent DE 197 36 514 C, the coating is generated bytargeted oxidation of the iron materials that are to be treated. Here,the parts to be heat-treated are oxidized in a defined manner during thehardening process (holding phase) so as to be simultaneously hardenedand blackened in the heat-treatment furnace. During the course of theheat-treatment process, an oxidizing component (oxidant) is fed into thetreatment chamber. A furnace gas atmosphere that has an oxidizing effecton iron is established after a brief time and a continuous iron oxidelayer is spontaneously formed on the surface of the material, a processwhich can be expressed by the following chemical reaction:Fe+H₂O

FeO+H₂

The barrier layer formed prevents an undesired de-carburization of theedge zone of the alloy. Due to the defined furnace gas atmosphere thathas been set, a certain PH₂O-to-PH₂ ratio is established that exerts apositive influence on the iron oxide structure and on the growth rate ofthe oxide. Oxide layers with a thickness of a few micrometers (maximumof 10 μm) have proven to be particularly advantageous.

The heat treatment and the oxidation of the parts take place in separateprocess steps in the shared treatment chamber. This method, for thefirst time, allows an essentially environmentally friendly production ofdecorative and corrosion-proof coatings.

The present invention is based on the objective of improving the methodof this generic type in terms of the reproducibility of the productionof corrosion-resistant coatings that cover and adhere well on metalparts containing iron and of improving the cost effectiveness of theprocess.

According to the invention, this objective is achieved on the basis ofthe above-mentioned process according to the invention in that theoxygen partial pressure is regulated in such a way that the oxygen probeindicates a probe voltage in the range from 890 mV to 940 mV.

The probe voltage is kept at a value in the range from 890 mV to 940 mVduring the oxidization phase within a temperature range of 750° C. to950° C. [1382° F. to 1742° F.]. Here, the probe voltage to beestablished is based on the following dependence on the partial pressureratio (PH₂O/PH₂):voltage (mV)=(LOG(PH₂O/PH₂)−13027/T+3.2906)·T·0.0992 (temperature inKelvin)

The probe voltage is a measure of the oxidation potential (oxygenactivity) in the treatment chamber. It has been surprisingly found that,in the case of metal parts containing iron, an optimally adhesive anddense oxide layer is obtained within this narrow process window. Thiscan be ascribed to the degree of disarrangement of the iron oxide thatis formed in this process. Within the temperature range cited and in thecase of the oxygen partial pressures defined by the probe voltage, thisis wücstite (Fe_(1-y)O). The degree of disarrangement (y) of the wüstitegenerated by the method according to the invention lies in the rangefrom 0.05 to 0.12, which promotes the formation of a dense oxidationlayer that adheres well.

Moreover, at an oxygen partial pressure within a range as defined by thecited probe voltage range, the duration of the oxidation can beshortened so that this also translates into a “rise in productivity” incomparison to the prior-art method.

Along with this, the shortening of the duration of the oxidation alsoleads to a reduction of the otherwise frequently observed partialde-carburization of the edge of the material, a phenomenon caused by areaction of the oxygen in the area of the grain boundaries(intragranular attack).

Furthermore, during the heating of the treatment chamber that followsthe oxidation phase and that is meant for a subsequent batch to betreated, the requisite carbon level of the endothermic atmosphere isbuilt up more quickly.

The FeO oxide layer formed has a uniform structure, it adheres well, itis scratch-resistant as well as free of bubbles or contact areas. Withina short oxidation time (typically less than 20 minutes), an oxide layer(FeO layer) that is less than 10 μm thick is formed due to the definedoxygen activity. The layer thus built up is compact, covers well, can beinterlocked well with the base material and has an attractive blackcolor.

With an oxygen partial pressure that yields a probe voltage of more than940 mV, the build-up of the oxide layer is too slow, with the resultthat pronounced decarburization of parts of the edge of the materialoccurs. In the case of an excessive oxygen partial pressure that resultsin a probe voltage of less than 890 mV, in contrast, an undesired thickoxidation layer is formed that can chip off or form bubbles in places.This defect can also be noticed in that oxide particles can be wipedoff. The cause lies in an excessively thick oxide layer that is formedat a higher speed (mm per hour) owing to the higher degree ofdisarrangement.

The oxidant is normally distilled water, although other oxidants such asair or oxygen should not be ruled out.

Preferably, the oxygen partial pressure is regulated in such a way thatthe oxygen probe indicates a probe voltage ranging from 900 mV to 925mV. This range of the probe voltage has proven to be a particularlysuitable compromise between optimal oxide properties and minimal edgede-carburization on the one hand, and the desired black color on theother hand.

Preference is given to a variant of the method in which a first constantpartial amount of the oxidant and a second partial amount of the oxidantare fed into the treatment chamber, whereby the second partial amount isfed in a regulated manner and as a function of the probe voltage. Here,only a partial amount of the total quantity of oxidant is fed into thetreatment chamber in a regulated manner (regulated amount) while theother partial amount—which can be the smaller partial amount—does nothave to be regulated (base amount). This serves to simplify theregulation and to reduce the complexity of the equipment needed.

Another improvement of this variant of the method consists in feeding inthe second partial amount intermittently. The intermittent feed of theoxidant is conducive to a better regulation constancy. As a result, thetarget value to be regulated hardly fluctuates, so that the deviationfrom the target voltage is merely a few mV (millivolts), for instance,±1 millivolt.

Preference is given to a procedure in which a nitrogen stream is fedinto the treatment chamber, and this stream is set as a function of thecomposition of the atmosphere in the treatment chamber. The retentiontime of the gas components is influenced by means of the variablenitrogen stream.

In this context, care should be taken to ensure that the retention timeof the components in the gas phase is reflected in the course of theprobe voltage over time, which is also decisive for the outcome of theoxide formation. Particularly in the first minutes of a gas phase changefrom a carburization phase to the oxidation phase, the nucleation of theoxide on the metallic iron surface can be influenced by the gas phase.If the oxygen partial pressure drops too rapidly, the number of oxidenuclei formed is low so that the oxide color makes more of a transitiontowards gray. Therefore, the speed at which the probe voltage is loweredfrom the carburization phase (at a probe voltage of, for example, 1060mV) to the oxidation phase (at a probe voltage of, for example, 920 mV)influences the outcome of the oxide properties and has to be optimizedby experiments as a function of the material and shape of the part.

An optimal result in terms of the properties of the oxide layer on themetal parts is obtained when the maximum oxidation temperature is 830°C. [1526° F.]. Here, the optimal probe voltage is around 920 mV.Lowering the oxidation temperature has a positive effect on the oxideproperties. Here, the specific materials that have actually beenemployed as well as the desired nucleus hardness play a decisive role.

But it is not only the amount of oxidant fed in that is important forthe quality of the oxide layer formed, but rather also the amount ofnitrogen that is fed into the treatment chamber together with theoxidant. It has been found that nitrogen volumes of 6 m³ to 12 m³ perhour are optimal. By varying this volume of nitrogen, the color anduniformity of the oxide to be formed can be improved. Naturally, thenitrogen quantity is also based on the free volume present in thetreatment chamber and, for safety reasons, must not fall below a certainminimum amount.

It has proven to be especially advantageous to raise the carburizationtemperature in the treatment chamber after completion of the oxidationphase and to establish a carbon level of 0.2% to 0.3% C in the treatmentchamber. Consequently, the water can react faster and more efficientlywith the lubrication gas (hydrocarbons, for example, natural gas orpropane), as a result of which the oxygen partial pressure dropsconsiderably, or else the probe voltage rises. A favorable voltage valueat which a new batch can be conveyed into the treatment chamber is 1000mV. Starting at this voltage, there is no longer a difference from thecommonly employed batches which have not been preceded by oxidationsince here air always enters the treatment chamber when the furnacedoors are opened, as a result of which the probe voltage drops to lowvalues. Efforts are aimed at achieving the fastest possible change fromoxidation to a neutral or carburizing endothermic atmosphere phase. Theresult of this special modality of formation is that formation times of10 to 15 minutes are commonly achieved, which translates into only aslight prolongation of the treatment duration.

The method according to the invention is particularly suitable for theproduction of coatings in the discontinuous mode of operation. However,a variant of the method that has also proved to be advantageous is onein which the metal parts are continuously moved through the treatmentchamber so that they can be hardened and so that the oxide layer can beproduced.

The invention will be described in greater detail below with referenceto embodiments.

EMBODIMENT 1

In a multipurpose chamber furnace that operates discontinuously (brandname: TQ 10), tool-holding fixtures made of case-hardened steel 16 MnCr5 were placed and affixed in charging racks. These were then moved intothe heating and treatment chamber and brought to the desiredtemperature. An endothermic atmosphere (20% CO, 40% H₂ and the rest N₂)made from natural gas continuously flowed through the multipurposechamber furnace as the carrier gas during the entire carburizationphase. The carbon level of the gas phase was regulated through theaddition of hydrocarbons until the desired edge carbon content of 0.75%C and the requisite case-hardening depth of 1.20 mm had been reached.The entire process was regulated or controlled by means of the existingprocess control system. At a case-hardening depth of 1.10 mm, thefurnace temperature was lowered to 830° C. [1526° F.] and the carbonlevel was kept constant at 0.75 mm.

Shortly before the end of the treatment time and before thecase-hardening depth of 1.20 mm had been reached, water—as theoxidant—together with nitrogen were injected into the treatment chamberof the furnace. The volumetric flow rates of the oxidizing reactant andof the nitrogen were set in such a way that the PH₂O-to-PH₂ ratioestablished in the treatment chamber of the furnace yielded a constantprobe voltage of 920 mV. Over the course of 12 minutes, a thin, adhesiveFeO oxide layer was formed having a thickness of 5 μm and a blue-blackcolor.

The following criteria were fulfilled:

-   -   1. adhesive, black oxide layer    -   2. no light-colored areas caused by contact areas    -   3. no bubble formation or chipping off of the oxide layer    -   4. surface hardness prior to the annealing: 57 to 58 HRC    -   5. oxide layer still adheres after the annealing: (180° C. [356°        F.], 3 hours)    -   6. surface hardness after the annealing: 52 to 54 HRC    -   7. no edge de-carburization    -   8. overall assessment: no defects!

The C level of the furnace gas atmosphere desired after the oxidationtreatment was reached once again after a brief time, so that nooxidation or de-carburization of the following batch occurred. No watercondensation was observed on the cold spots of the antechamber or in theexhaust gas lines. The reproducibility of the oxidation processes wasflawless.

The corrosion resistance of these parts oxidized by means of the methodaccording to the invention is better than that of the conventionallycoated, burnished parts, so that the parts can be stored for longerperiods of time without the formation of detrimental rust. It is thenpossible to dispense with oiling the oxidized parts or applying afluid—as is done in burnishing—without this giving rise to a rust film.Due to the elimination of the surface-active liquids, the parts do nothave to undergo complicated cleaning procedures during the subsequentmechanical processing operations; the absence of any residual liquidadhering to the parts makes it possible to use the drilling emulsion fora longer time since no contamination is possible.

Prior to the burnishing process, the hardened parts have to bedepassivated by means of sandblasting so that a layer having a thicknessof 1 μm to 2 μm can be formed. During sandblasting, grains of sand andparticles easily accumulate in blind holes or fissures and they damagethe drills or cutting tools, thus considerably reducing the service lifeof these tools, so that demanding, painstaking cleaning operations haveto be carried out for the parts. These costs and time losses were alleliminated through the use of the coating method according to theinvention so that the parts could be quickly further processed.

1. A method for producing an oxide layer on metal parts, the metal partsare heat-treated in a treatment chamber during a carburization phase attemperatures below 1100° C. [2012° F.] in an atmosphere containingcarbon monoxide and hydrogen and then, during an oxidation phase, theyare oxidized at an oxidation temperature in the range from 750° C. to950° C. [1382° F. to 1742° F.] in an atmosphere where a PH₂O-to-PH₂ratio between 0.3 and 10 has been established by feeding an oxidant intothe treatment chamber, a process is which an oxide layer is formed,whereby the oxygen partial pressure is determined by means of an oxygenprobe and regulated at least during the oxidation phase, the oxygenpartial pressure is regulated in such a way that the oxygen probeindicates a probe voltage in the range from 890 mV to 940 mV.
 2. Themethod according to claim 1, characterized in that the oxygen probeindicates a probe voltage in the range from 900 mV to 925 mV.
 3. Themethod according to claim 1 or 2, characterized in that a first constantpartial amount of the oxidant and a second partial amount of the oxidantare fed into the treatment chamber, whereby the second partial amount isfed in a regulated manner and as a function of the probe voltage.
 4. Themethod according to claim 3, characterized in that the second partialamount is fed in a regulated manner and as a function of the probevoltage.
 5. The method according to claim 3 or 4, characterized in thatthe second partial amount is fed in intermittently.
 6. The methodaccording to one of the preceding claims, characterized in that anitrogen stream is fed into the treatment chamber that is varied as afunction of the composition of the atmosphere in the treatment chamber.7. The method according to one of the preceding claims, characterized inthat the treatment chamber is brought to the carburization temperatureafter completion of the oxidation phase, and a carbon level in the rangefrom 0.2% to 0.3% C is established in the treatment chamber.
 8. Themethod according to claim 7, characterized in that the carbon level isregulated, whereby a propane gas stream fed into the treatment chamberis employed as the control quantity for the regulation.
 9. The methodaccording to one of the preceding claims, characterized in that theoxidation temperature is optimally 830° C. [1526° F.].
 10. The methodaccording to one of the preceding claims, characterized in that themetal parts are moved continuously through the treatment chamber so thatthey can be hardened and so that the oxide layer can be produced.